System and method for determining defrost power delivered by a defrost heater

ABSTRACT

An adaptive defrost control system of the present invention monitors an amount of current flowing through the defrost heater to calculate the amount of power delivered thereby. The circuit utilizes a thermistor to monitor the temperature rise of the electrical trace supplying current to the defrost heater to allow the controller to calculate an amount of power delivered thereby. A second thermistor may be used to compensate for a change in ambient temperature that might otherwise be attributed to a change in current flow through the power trace. A physical modification to the power trace to enhance the temperature rise characteristic at the point of placement of the thermistor enhances the accuracy of the calculation. A secondary current flow path around the branch of thermistor placement is also provided so as to not reduce the total current carrying capacity of the power trace.

FIELD OF THE INVENTION

The present invention relates generally to defrost heaters for consumerand commercial appliances, and more particularly to defrost cyclecontrol methods to control the defrost cycle in such appliances.

BACKGROUND OF THE INVENTION

Consumer and commercial refrigeration systems typically utilize acompressor driven refrigeration cycle to provide the cooling necessaryto maintain the internal temperature of the freezer compartment of arefrigerator or freezer at a particular selected temperature. In arefrigerator, a damper door is typically utilized to allow some of thecold air from the freezer compartment to flow into the fresh foodcompartment to maintain the fresh food compartment at a differentselected temperature. Such a compressor refrigeration system includes anevaporator positioned within or in close proximity to the freezercompartment. The evaporator is a heat exchanger through which is blownrecirculated air within the freezer compartment to reduce thetemperature thereof. This heat exchanger is typically a coiledarrangement of refrigeration line with fins connected thereto toincrease the surface area and therefore the efficiency of the heattransfer.

Unfortunately, due to the extremely cold temperature of the evaporator,any moisture in the air in the freezer compartment tends to freeze onthe evaporator during operation. This frost build-up on the evaporatorreduces the amount of surface area over which the air may be blown.Indeed, if this condition were allowed to continue, the frost wouldbuild to ice that would completely block off the air flow paths throughthe evaporator. This greatly reduces the efficiency of the refrigerationsystem as the heat transfer is greatly reduced. As a result, thecompressor is required to run, potentially continuously, in order to tryto maintain the temperature of the freezer compartment at theappropriate level. This greatly increases the cost of operating thisrefrigeration equipment, and reduces the owner's satisfaction as thenoise generated by the continuously running compressor may well beannoying to the consumer.

Recognizing these problems, many modern refrigeration systems include adefrost heater that is placed in proximity to the evaporator heatexchanger. This defrost heater is controlled by a controller, which maytake the form of an electromechanical defrost timer or may utilize amicroprocessor or microcontroller. In any event, the defrost heater isutilized to provide a small amount of heating of the evaporator coilafter the refrigeration cycle has stopped to defrost the heat exchanger,i.e. melt the ice or frost that may have formed thereon during therefrigeration cycle. Through the use of such a defrost heater, theefficiency of the refrigeration system is maintained at a high level soas to reduce the cost of operation of the appliance and to increase theowner's satisfaction as the refrigeration cycles may be kept short dueto the high efficiency energy transfer through the defrosted evaporatorcoils.

Unfortunately, since the evaporator coils are located physically withinor in thermal proximity to the interior of the freezer compartment, theheating from the defrost heater tends to increase the temperature in thefreezer compartment. Since a rough rule of thumb is that twice theamount of cooling is required to remove each unit of heat from thefreezer compartment, the defrost heater should be operated for the leastamount of time to provide the defrosting of the evaporator coils.Otherwise, the temperature of the freezer compartment may beunnecessarily increased, which increases the amount of cooling necessaryto remove this unnecessary heat in the next refrigeration cycle.

Electromechanical defrost timers are used in an attempt to limit theamount of time that the defrost heater is energized. This energizationtime is set based on experimentations with the particular appliances todetermine how long the defrost heater should operate to remove a typicalamount of frost that would build up during a typical refrigeration cyclewithin that particular appliance. This timing is selected to optimallyremove this amount of frost. If the defrost heater is run for too long,unnecessary heating of the freezer compartment occurs without any addedbenefit to the removal of frost from the evaporator coils. If thedefrost heater is not run for a long enough period, the residual frostthat remains on the evaporator coils will tend to build up over time sothat eventually the benefit of the defrost cycle is completelyovershadowed by the increasing accumulation of frost on the evaporatorcoils. Therefore, a large amount of testing and calculations go into theselection of the appropriate defrost times. While microprocessorcontrols may provide for adaptive defrost control, the utilization of atimed cycle is still the hallmark of the control of the defrost cycle.

Unfortunately, while the defrost control cycles are carefully designedand tested to optimize the operation thereof, once the particularappliance has been installed at a consumer or commercial location, otherfactors that typically occur during normal operation may have a largeeffect on the efficiency of the defrost heater operation. Specifically,the calculation and testing to program the adaptive control to providean optimized time for operation of the refrigeration and defrost cycleis based on a given voltage to be applied to the defrost heater. Thisadaptive control utilizes a temperature sensing element on theevaporator, and monitors the temperature and time of the defrost cycle.That is, after the refrigeration cycle when the defrost heater isenergized, the controller monitors the temperature of the evaporator.Once the temperature reaches a predetermined point, the controllerassumes that it has been fully defrosted. The time that this processtakes is then used by the controller to adjust the subsequentrefrigeration cycle, hence the “adaptive” nature of the controller.

Since defrost heaters are typically resistive heating elements, theamount of power that is dissipated, i.e. turned into heat, through theresistive heating element is affected by variations in the line voltage.As such, a the typical controller assumes that a known amount of heatingis provided during the particular defrost cycle based on a known linevoltage and the value of the resistive heating element. Therefore, ifall of these were to remain constant, the defrost cycle time on whichthe adaptive controller bases the next refrigeration cycle would operatein the field as it does in the lab.

Unfortunately, the line voltage at a consumer or commercial installationmay vary greatly from that of the laboratory in which the cycle time wasdetermined, and may, in fact, vary greatly over its operational life.These variations are not symptomatic of any problem in the electricaldistribution system, but are, typically, within the specifications forpower delivery. Unfortunately, as the input line voltage varies, so doesthe amount of heat generated or power delivered by the defrost heater.This will directly affect the time it takes to raise the temperature ofthe evaporator to the set point under identical frost conditions.

A line voltage lower than that utilized in the development of theadaptive cycles will result in less heat being delivered to melt thefrost and ice from the evaporator, and therefore will result in a longertime needed to raise the temperature of the evaporator to thepredetermined level. As such, the adaptive controller may well reducethe refrigeration cycle time thinking that the last refrigeration cycletime severely frosted the evaporator. Similarly, an increase in the linevoltage over that utilized in the development of the adaptive cycleswill result in more heat being delivered by the heater. As a result, thetime to reach the predetermined temperature of the evaporator will beshorter. This will cause the adaptive controller to believe that verylittle frost accumulated on the previous refrigeration cycle. As aresult, the controller will likely lengthen the next refrigerationcycle. These variations will increase the cost of ownership of thisappliance since the adaptive control can no longer vary the cycles totheir most efficient.

As an example, in a typical defrost heater during a typical defrostcycle, the amount of power delivered when the line voltage is a nominal117 volts AC is approximately 400 watts. If the line voltage increases,within its specifications, to 132 volts AC, the amount of powerdelivered during the same defrost cycle will increase to approximately600 watts. Conversely, if the line voltage were to be at the low end ofthe specification, for example 102 volts AC, the amount of powerdelivered during the same defrost cycle will drop to only approximately350 watts. As this example illustrates, the mere variation in the linevoltage over its specified range results in vastly different amounts ofheating that will be generated by the defrost heater, and therefore thetime it takes for the defrost cycle to raise the temperature of theevaporator will vary widely.

There exists, therefore, a need in the art for a defrost heater cyclecontrol that can determine the actual amount of heating during a defrostcycle so as to properly adjust the adaptive control in order to operatein an efficient manner. This system and method of the present inventionprovides such an adaptive defrost heater cycle control.

These and other advantages of the invention, as well as additionalinventive features, will be apparent from the description of theinvention provided herein.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a new and improved defrost heater controlsystem and method that overcomes one or more of the problems existing inthe art. More particularly, the present invention provides a new andimproved defrost cycle control that monitors or calculates the defrostcycle to determine an actual amount of heating during the defrost cycleregardless of variations of the input line voltage. Still moreparticularly, the present invention provides a new and improved defrostcycle control that adapts the cycle control based upon an amount ofpower delivered to the defrost heater during the defrost cycle.

In one embodiment of the present invention the defrost heater controlmonitors an amount of current delivered to the defrost heater during thedefrost cycle so that an amount of power delivered by the defrost heatermay be continuously calculated to account for the length of the defrostcycle. A current sensor is used to monitor the amount of currentprovided to the defrost heater, which current will vary depending on theline voltage. The square of this monitored current is multiplied by theknown resistance of the defrost heater to calculate an instantaneouspower supplied by the defrost heater. This instantaneous power isaccumulated over the defrost cycle until the defrost cycle isterminated. In one embodiment, the duration of each defrost cycle may beindependently calculated during the defrost cycle itself, and takes intoconsideration the varying input line voltage so that only an appropriateamount of defrosting occurs during any particular cycle.

In a preferred embodiment of the present invention, the current sensoris a thermistor. Preferably, the thermistor is placed on the electricaltrace on the defrost heater power control board. The adaptive defrostcontroller monitors the temperature rise of this thermistor over time todetermine the amount of current supplied therethrough. Preferably, asecond thermistor is utilized off the power trace so as to allow thefactors of the ambient temperature to be eliminated.

Other aspects, objectives and advantages of the invention will becomemore apparent from the following detailed description when taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings incorporated in and forming a part of thespecification illustrate several aspects of the present invention and,together with the description, serve to explain the principles of theinvention. In the drawings:

FIG. 1 is a simplified single line schematic diagram of one embodimentof a defrost heater control circuit constructed in accordance with oneembodiment of the present invention;

FIG. 2 is a simplified single line schematic diagram of an alternateembodiment of a defrost heater control circuit constructed in accordancewith the teachings of the present invention;

FIG. 3 is a simplified single line schematic diagram of a furtheralternate embodiment of a defrost heater control circuit constructed inaccordance with the teachings of the present invention illustratingadditionally physical features thereof; and

FIG. 4 illustrates a portion of a defrost heater power trace andelectrical connection pads for a thermistor that may be used in oneembodiment of the present invention, such as in the circuit illustratedin FIG. 3.

While the invention will be described in connection with certainpreferred embodiments, there is no intent to limit it to thoseembodiments. On the contrary, the intent is to cover all alternatives,modifications and equivalents as included within the spirit and scope ofthe invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a simplified single line schematic diagram of anadaptive defrost control circuit 10 constructed in accordance with theteachings of the present invention. Specifically, the adaptive defrostcontrol (ADC) circuit utilizes an ADC controller 12, which is preferablya microcontroller, microprocessor, programmable logic device, etc., thatmay adaptively change the refrigeration cycle time based upon how longthe defrost heater 14 is energized before the evaporator temperaturereaches a predetermined temperature. The ADC controller 12 controls thestart and stop of a defrost cycle by closing or opening a power controlswitch 16. This is well known in the art, such a power control switch 16may take the form of an electromechanical relay, a power switchingsemiconductor, etc.

Once the ADC controller 12 has closed the switch 16, AC power 18,typically from the utility, is applied to the defrost heater 14. Asdiscussed above, the voltage from the source 18 may vary widely duringnormal operation, therefore, delivering varying amounts of current tothe defrost heater 14. Preferably, the defrost heater 14 is a resistiveheating element, the power delivered by which may be calculated as theamount of current provided therethrough varies over the defrost cycle.Through calculation and experimental testing in the developmentlaboratory, the amount of power necessary to remove the frost build upfrom an evaporator in the freezer compartment of a particular applianceis well understood, as is the formation of such frost during therefrigeration cycle. As such, the ADC controller 12 is programmed withthese values. The ADC controller 12 utilizes this information todetermine the duration of the defrost cycle and adjust the refrigerationcycle to ensure continued efficient operation of the refrigerationsystem.

In view of varying voltage from source 18 during the defrost cycle, thesystem of the present invention utilizes a current sensor 20 placed incircuit with the defrost heater 14 to monitor an amount of instantaneouscurrent being supplied through the defrost heater 14. The ADC controller12 then calculates the total amount of power supplied by accumulatingthe instantaneous power during the defrost cycle. Once an appropriateamount of power has been delivered by the defrost heater 14 as typicallydetermined by sensing the temperature of the evaporator, the ADCcontroller 12 commands the switching element 16 to open to stop the flowof current to the heater 14. As such, a fixed defrost cycle time neednot be used as was the case with the electromechanical timers.

While any current sensing device could be utilized in the system of thepresent invention, in the consumer and commercial refrigeration market,cost sensitivity for individual components is high. As such, while acurrent transformer (CT) could be used as the current sensing device 20,the cost of such current transformers may be prohibitive to the overallcost of the design for a consumer or commercial refrigeration unit. Assuch, a lower cost alternative needed to be found.

Recognizing that thermistors are substantially less expensive thancurrent transformers, on the order of 7 to 8 cents apiece andrecognizing that the ADC controller 12 could calculate current based ona measured temperature rise of the power trace on the defrost heaterpower control board, it was decided to attempt to utilize theseinexpensive thermal sensing components to determine the current flowingto the defrost heater. FIG. 2 illustrates such a circuit utilizing athermistor 22 to provide the current sensing function for use by the ADCcontroller 12.

That is, the thermistor 22 is placed on the power trace 26 for thedefrost heater 14. This power trace 26 has a known resistance, andtherefore the flow of current therethrough will result in a temperaturerise thereof. This temperature rise is detected by thermistor 22. TheADC controller 12 then performs a calculation that takes into accountthe temperature rise over time to calculate the amount of currentflowing through the trace 26, and therefore the amount of currentflowing through defrost heater 14. This current value is then utilizedto calculate the instantaneous power delivered by the defrost heater 14,which values are accumulated until the desired temperature of theevaporator is reached, i.e. until the proper amount of power has beendelivered to melt the frost and/or ice that may have accumulated on theevaporator in the prior refrigeration cycle. At such a point, the ADCcontroller 12 will command the switching element 16 to terminate thedefrost cycle. While various types of thermistors may be used, apreferred embodiment of the present invention utilizes surface mountthermistors, such as NTC thermistors, Linear PTC thermistors, etc.

To increase the accuracy of the calculation of the current flowingthrough the power trace 26 as sensed by the temperature rise bythermistor 22, a second thermistor 24 is utilized to provide anindication of the ambient temperature of the control circuitry. In otherwords, the second thermistor 24 is used to provide a correcting factorbased upon a change in the ambient temperature that may occur due to theheating caused by the defrost heater 14. Such an increase in ambienttemperature will also be sensed by thermistor 22. If this rise inambient temperature is not compensated, the ADC controller 12 may thinkthat the increased temperature sensed by thermistor 22 is due to anincreased current flow through the defrost heater 14 as measured ontrace 26.

To prevent this erroneous situation from occurring, the adaptive defrostcontroller 12 compensates the temperature sensed by thermistor 22 by thetemperature differential sensed by thermistor 24. This net temperaturerise, therefore, is due only to the temperature rise of the trace 26. Itis this net temperature rise that is used by the ADC controller 12 tocalculate the amount of current flowing through the defrost heater 14and the trace 26.

While the utilization of a thermistor 22 to sense the temperature riseof the power trace 26 solves both the variation in power delivered andcost of sensing current flowing the defrost heater 14, typical powertraces, and indeed wiring for power devices in general, have a very lowlinear resistance. This is particularly true in configurations as aretypically used in such circuitry. As such, and to enhance the ability ofthe thermistor 22 to actually sense a temperature rise, a preferredembodiment of the present invention utilizes a physical layout thatprovides such enhanced sensing ability.

One such circuit layout that will provide this enhanced sensitivity isillustrated in FIG. 3. While the circuit components remain the same, aphysical configuration of the power trace 26 on which the thermistor 22is placed in thermal contact is chosen to enhance, or increase itslinear resistance so that the temperature rise to be sensed by thethermistor may be provided with better resolution. Since a typical powertrace is fairly wide, it was determined that if the power trace at thepoint of thermistor 22 placement could be narrowed, the thermistor 22would be able to better sense a temperature rise.

However, it was also recognized that the width of the power trace iscalculated based on transient and maximum current carrying capability.As such, it was important not to minimize the ability of the powercircuit itself to carry such maximum currents. In view of this, asecondary current carrying path 28 was added into the circuit to providean alternate path for current flow at the point of thermistor placement.Such an alternate path 28 insures the ability of the overall circuit tocarry such high transient currents.

The provision of an alternate path would also allow the current actuallyflowing through the defrost heater 14 to take different paths, andtherefore the temperature rise sensed by thermistor 22 would not berepresentative of the actual current flowing through the defrost heater14. To overcome this problem, the length, and therefore the resistance,of the alternate path 28 was made to ensure that under normal operatingconditions, the majority of the current flowing through the defrostheater 14 will flow through the power trace 26 path as opposed to thealternate path 28. In this way, the calculation within the ADCcontroller 12 maintains accuracy. Indeed, in one embodiment of thepresent invention the current ratio flow between paths 26 and 28 mayalso be taken into account with the ADC 12 to ensure that the fullcurrent flow is taken into consideration.

FIG. 4 illustrates an exemplary embodiment of a physical implementationof a section of the power trace 30 of FIG. 3. As may be seen in thisFIG. 4, the width of the power trace 30 is greatly reduced in the branch26 over which the thermistor will be positioned to sense temperaturerise thereacross. This FIG. 4 also illustrates the alternate path 28that is provided to ensure that the overall current carrying capabilityof the trace 30 is not lessened by the modification to provide anenhanced thermal sensing capability through branch 26. As may also beseen in this FIG. 4, attachment pads 32, 34 are provided on either sideof branch 26 for surface mount placement of the thermistor 22 (see FIG.3). the electrical connection from attachment pad 34 is jumpered overbranch 26 to trace 36. The actual configuration of the alternate branch28 may vary widely, but preferably provides an increased resistance tocurrent flow so as to maximize the ability of the thermistor placed overbranch 26 to sense a temperature rise. Of course, the resistance ofbranch 26 is known and utilized by the ADC controller 12 to perform thetemperature rise to current calculation.

All references, including publications, patent applications, and patentscited herein are hereby incorporated by reference to the same extent asif each reference were individually and specifically indicated to beincorporated by reference and were set forth in its entirety herein.

The use of the terms “a” and “an” and “the” and similar referents in thecontext of describing the invention (especially in the context of thefollowing claims) is to be construed to cover both the singular and theplural, unless otherwise indicated herein or clearly contradicted bycontext. The terms “comprising,” “having,” “including,” and “containing”are to be construed as open-ended terms (i.e., meaning “including, butnot limited to,”) unless otherwise noted. Recitation of ranges of valuesherein are merely intended to serve as a shorthand method of referringindividually to each separate value falling within the range, unlessotherwise indicated herein, and each separate value is incorporated intothe specification as if it were individually recited herein. All methodsdescribed herein can be performed in any suitable order unless otherwiseindicated herein or otherwise clearly contradicted by context. The useof any and all examples, or exemplary language (e.g., “such as”)provided herein, is intended merely to better illuminate the inventionand does not pose a limitation on the scope of the invention unlessotherwise claimed. No language in the specification should be construedas indicating any non-claimed element as essential to the practice ofthe invention.

Preferred embodiments of this invention are described herein, includingthe best mode known to the inventors for carrying out the invention.Variations of those preferred embodiments may become apparent to thoseof ordinary skill in the art upon reading the foregoing description. Theinventors expect skilled artisans to employ such variations asappropriate, and the inventors intend for the invention to be practicedotherwise than as specifically described herein. Accordingly, thisinvention includes all modifications and equivalents of the subjectmatter recited in the claims appended hereto as permitted by applicablelaw. Moreover, any combination of the above-described elements in allpossible variations thereof is encompassed by the invention unlessotherwise indicated herein or otherwise clearly contradicted by context.

1. A system for determining an amount of power delivered by a defrostheater, comprising: an adaptive defrost control (ADC) controller; adefrost heater; and a sensing element from which an amount of currentflowing through the defrost heater may be determined; and wherein theADC controller monitors the sensing element over time to determine anamount of power supplied by the defrost heater during a defrost cycle.2. The system of claim 1, wherein the defrost heater is aresistance-type heating element.
 3. The system of claim 1, wherein thesensing element is a current transformer.
 4. The system of claim 1,wherein the sensing element is a first thermistor.
 5. The system ofclaim 4, wherein current is supplied to the defrost heater via a powertrace on a printed circuit board, and wherein the first thermistor ispositioned in thermal communication with the power trace on the printedcircuit board.
 6. The system of claim 5, wherein the power trace isdivided into a first lower resistance branch and a second higherresistance branch, and wherein the first thermistor is positioned inthermal communication with the first lower resistance branch.
 7. Thesystem of claim 6, wherein the second higher resistance branch is longerthan the first lower resistance branch.
 8. The system of claim 4,further comprising a second thermistor positioned to sense an ambienttemperature in proximity to the first thermistor, and wherein the ADCcontroller monitors the second thermistor to compensate for changes inthe ambient temperature.
 9. The system of claim 8, wherein the ADCcontroller subtracts the temperature reading from the second thermistorfrom the temperature reading from the first thermistor to determine thetemperature rise resulting from current flow through the power trace.10. The system of claim 5, wherein the ADC controller is programmed witha resistance of the power trace, and wherein the ADC controllercalculated the current as a function of temperature rise of the powertrace over time.
 11. A system for determining an amount of powerdelivered by a defrost heater in a refrigeration system, comprising: anadaptive defrost control (ADC) controller; a sensing element from whichan amount of current flowing through the defrost heater may bedetermined; and wherein the ADC controller monitors the sensing elementover a defrost cycle to determine an amount of power supplied by thedefrost heater during the defrost cycle, and wherein the ADC controlleradjusts a subsequent refrigeration cycle based on an amount of elapsedtime of the defrost cycle adjusted for the amount of power supplied bythe defrost heater.
 12. The system of claim 11, wherein the sensingelement is a first temperature sensing element positioned on a trace ofa control board that supplies power to the defrost heater.
 13. Thesystem of claim 12, wherein the first temperature sensing element is athermistor.
 14. The system of claim 12, wherein the power trace isconfigured to provide a narrower width portion at a location where thetemperature sensing element is positioned.
 15. The system of claim 14,wherein the power trace is configured to provide a secondary path aroundthe narrower width portion.
 16. The system of claim 15, wherein thenarrower width portion has a lower resistance than the secondary path.17. The system of claim 11, further comprising a second temperaturesensing element positioned to sense ambient temperature in proximity tothe first temperature sensing element.
 18. The system of claim 17,wherein the ADC controller subtracts an output from the secondtemperature sensing element from an output from the first temperaturesensing element to remove affects of changes in ambient temperature. 19.The system of claim 17, wherein the second temperature sensing elementis a thermistor.
 20. A system for adapting a refrigeration cycle timefor changes in voltage from a supply to a defrost heater, comprising: anadaptive defrost control (ADC) controller; a thermistor positioned inproximity to a power trace supplying power to the defrost heater; andwherein the ADC controller monitors the thermistor to determine atemperature rise of the power trace, and wherein the ADC controllercalculates a current flowing though the power trace based on thetemperature rise over time, the ADC controller calculating aninstantaneous amount of power supplied by the defrost heater based onthe current and a known resistance of the defrost heater, and whereinthe ADC controller adjusts a subsequent refrigeration cycle based on atime of the defrost cycle and the power supplied by the defrost heater.